1. Field of the Invention
The present invention relates to a mode-locked laser apparatus for outputting pulsed laser light by generating oscillating light from a solid-state laser medium by emission of excitation light from an excitation light source and by mode-locking.
2. Description of the Related Art
Conventionally, a mode-locking method (mode synchronization method) has been adopted as a technique for continuously generating high-speed repetitive optical pulse trains. In the mode-locking method, a solid-state laser medium placed in a resonator (resonant cavity or laser cavity) is excited (pumped) by a semiconductor laser or the like and the phases of a multiplicity of longitudinal oscillation modes are locked together. As the mode-locking method, there are an active method and a passive method. In the active method, an optical modulator is inserted into the resonator to provide loss modulation. In the passive method, a saturable absorber, of which the absorption coefficient nonlinearly changes relative to the intensity of incident light, is inserted into the resonator to passively achieve mode-locking. A mode-locked laser apparatus according to the passive method using the saturable absorber has been proposed, for example, in “Diode-pumped mode-locked Yb3+:Y2O3 ceramic laser”, T. Yanagitani et al., OPTICS EXPRESS, Vol. 11, No. 22, pp. 2911-2916, 2003, International Patent Publication No. WO00/45480 or the like.
When a solid-state laser medium doped with Yb (ytterbium) is used as the solid-state laser medium, light with the maximum oscillation wavelength (maximum peak wavelength in a fluorescence spectrum) of the solid-state laser medium is generally used as output light. For example, in the case of Yb:YAG (yttrium aluminum garnet), light with a wavelength of 1030 nm is used as output light (please refer to “60-W average power in 810-fs pulses from a thin-disk Yb:YAG laser”, E. Innerhofer et al., OPTICS LETTERS, Vol. 28, No. 5, pp. 367-369, 2003). The maximum oscillation wavelength is a wavelength obtained in a four-level system. However, since a laser lower level is close to a ground level, the four-level system functions as a three-level system. Therefore, electrons distributed in the laser lower level reabsorb oscillating light, thereby causing reabsorption loss. Hence, there is a problem that laser oscillation efficiency sharply drops. In FIG. 3, an absorption spectrum of Yb:YAG (solid line) and a fluorescence spectrum of Yb:YAG (broken line) are illustrated (FIG. 3 is cited from “Directly Single-Diode-Pumped Continuous-Wave Yb3:YAG Laser Tunable in the 1047 1051-nm Wavelength Range”, Valerii V. Ter-Mikirtychev and Viktor A. Fromzel, APPLIED OPTICS, Vol. 39, Issue 27, pp. 4964-4969, 2000). As illustrated in FIG. 3, a peak A of the intensity of absorption, representing reabsorption loss of the oscillating light as described above, is present at the wavelength of 1030 nm, at which the intensity of fluorescence indicates a peak Fmax of the intensity of fluorescence. Therefore, the laser oscillation efficiency sharply drops. Hence, for the purpose of preventing such reabsorption loss, it is necessary to suppress reabsorption of the oscillating light by filling the laser upper level with electrons by high-density excitation.
When a semiconductor laser is used as excitation light, the output of a semiconductor laser that is available in the market is limited. Therefore, it is necessary to reduce a beam diameter in a solid-state laser medium to increase the density of the laser and to increase oscillation efficiency by increasing overlap between excitation light and oscillating light. The overlap between the excitation light and the oscillating light in the solid-state laser medium should be increased because if the overlap is small, the oscillation efficiency of the laser drops and there is a risk that the high-density excitation is not sufficiently achieved. Specifically, if the beam diameter of excitation light is less than that of oscillating light in the solid-state laser medium, reabsorption loss increases in a portion at which the excitation light is not present, and thereby the oscillation efficiency of the laser drops. In contrast, if the beam diameter of excitation light is greater than that of oscillating light in the solid-state laser medium, a portion at which the oscillating light is not present, in other words, a portion that does not contribute to oscillation, is excessively excited, and thereby the oscillation efficiency of the laser drops. Consequently, there is a risk that the high-density excitation is not sufficiently achieved.
Therefore, for performing high-efficiency laser oscillation, it is necessary to form a small beam waist of excitation light and a small beam waist of oscillating light in the solid-state laser medium and to increase overlap between the excitation light and the oscillating light. Therefore, a resonator is generally designed in such a manner.
However, such restrictions in design prevent reduction in the number of parts of the mode-locked laser apparatus and simplification of the structure of the mode-locked laser apparatus. If the structure of the apparatus is not simplified, there is a problem that it is impossible to reduce the cost of the apparatus.